A website could have obscured the fullscreen notification by using a URL with a scheme handled by an external program, such as a mailto URL. This could have led to user confusion and possible spoofing attacks. This vulnerability affects Firefox < 115, Firefox ESR < 102.13, and Thunderbird < 102.13.
https://www.mozilla.org/security/advisories/mfsa2023-24/
https://www.mozilla.org/security/advisories/mfsa2023-23/
https://www.mozilla.org/security/advisories/mfsa2023-22/
https://www.debian.org/security/2023/dsa-5451
https://www.debian.org/security/2023/dsa-5450
https://lists.debian.org/debian-lts-announce/2023/07/msg00015.html
https://lists.debian.org/debian-lts-announce/2023/07/msg00006.html